1. Field of Invention
This invention relates to an electro-deionization (EDI) water producing apparatus adapted to transfer ions in a liquid under the influence of an electric field, and more particularly, to such an EDI apparatus that utilizes stabilizing DC current.
2. Description of Related Art
Previously, various techniques have been developed to purify and isolate liquids or to obtain concentrated pools of specific ions or molecules from a liquid mixture, such as electrodialysis, liquid chromatography, membrane filtration, ion exchange, etc. Electro-deionization (EDI) is a technique that removes ionizable species from liquids using electrically-active media in an electrical potential to influence ion transport. The electrically-active media may alternatively collect and discharge ionizable species, or facilitate the transport of ions continuously by ionic or electronic substitution mechanisms.
The first apparatus and method for treating liquids by EDI was described by Kollsman in U.S. Pat. Nos. 2,689,826 and 2,815,320. Improved EDI systems have been shown in U.S. Pat. Nos. 4,925,541; 4,931,160 and 5,316,637. The typical structure of a module used for EDI includes alternating electroactive semi-permeable anion and cation ion exchange membranes in a stack mechanical sheet type structure.
An EDI apparatus having a helical configuration is shown in U.S. Pat. No. 6,190,528 (the '528 patent, which is based upon Chinese Patent No. 98223514) and whose entire disclosure is incorporated by reference herein. In the '528 patent, an insulated net-separating wall is positioned between a pair of anion and cation exchange membranes to form a special membrane bag type flow unit (hereinafter referred to a “first flow unit”). Each first flow unit is linked with at least one slot on a side wall of a central pipe, and is rolled to form a spiral wounded cylinder structure which centers on the central pipe. A conductive crust or shell is formed by winding metal outside the cylinder and electrically-active media (e.g., ion exchange resin) is inserted into the regions between the membrane bags to form a second flow unit. The product is enclosed by a housing with dome-shaped covers.
Previously, because of the electrode configuration, (for example Chinese Patent No. 98223514 which uses simple bridge-type rectification), energizing such EDI modules, unstable currents between the electrodes are experienced due to variances which include factors such as, but not limited to, unstable or fluctuating feedwater temperature, feedwater flow rate and feedwater quality, unstable or fluctuating EDI module input voltage, variances in construction of EDI modules themselves, etc., resulting in unstable product water quality. Thus, there remains a need for stabilizing these currents between the electrodes. There also remains a need for configuring a plurality of EDI modules with an electrical power interface while reducing the current requirement.
All references cited herein are incorporated herein by reference in their entireties.